Final answer:
The prefix that means decreased is 'hypo-,' commonly used in chemistry to denote fewer oxygen atoms in oxyanions compared to -ite anions, such as in hypochlorous acid.
Step-by-step explanation:
The prefix that means decreased is hypo-. This is often used in chemistry to indicate that an ion has one fewer oxygen atom compared to the -ite anion. For example, in the cases of oxyanions of halogens, the ion with one fewer oxygen atom than the -ite anion is given a hypo- prefix, as in hypochlorous acid (HClO), which has one fewer oxygen atom than chlorous acid (HClO2).